When a free electron approaches to a junction, in addition to the uniform electric field $\mathrm{E}$ facing it normally that keep thedrift velocity fixed. And the drift velocity depend on E, $\mathrm{e}, \tau, \mathrm{m}$ by the relation drift velocity
$$
\mathrm{V}_{\mathrm{d}}=\frac{\mathrm{eE} \tau}{\mathrm{m}}
$$
Then the result into accumulation of charges on the surface of wires at the junction. Which will affect the drift velocity because, it produce additional electric field. These fields change the direction of momentum.
Hence, the motion of a charge across junction is not momentum conserving.